Magnetoinductive flowmeters and measuring methods for magnetoinductive flowmeters of the type referred to above have been well known for some time and are employed in a wide variety of applications. The underlying principle of a magnetoinductive flowmeter for measuring the flow rate of a medium traveling through a measuring tube goes all the way back to Faraday who as early as 1832 proposed applying the principle of electrodynamic induction in the measurement of flow rates.
Faraday's law of induction postulates that when a flowing medium that contains charge carriers travels through a magnetic field, an electric field intensity is generated in the medium perpendicular to the direction of flow and to the magnetic field. Magnetoinductive flowmeters utilize Faraday's law of induction in that a magnet, usually consisting of two magnetic poles, each with a field coil, generates a magnetic field that contains a magnetic field component perpendicular to the direction of flow in the measuring tube. Within that magnetic field, each volume element of the medium traveling through the magnetic field and containing a particular number of charge carriers contributes the field intensity generated in it to the voltage collected by the measuring electrodes.
In the traditional magnetoinductive flowmeters, the measuring electrodes are so designed as to connect to the flowing medium either through conductive or capacitive coupling. Another particular feature of magnetoinductive flowmeters is the proportionality between the measured voltage and the flow rate of the medium as averaged across the diameter of the measuring tube, i.e. between the measured voltage and the volumetric flow.
In an actual flow-measuring operation, the magnetic field in a magnetoinductive flowmeter is usually reversed in periodically alternating fashion. Prior art has developed a variety of approaches to that effect. For example, magnetoinductive flow measurements can be achieved using an alternating field in which case the field coils of the magnet are typically connected directly to a sinusoidal 50 Hz alternating line-voltage source. However, the voltage generated by the flow between the measuring electrodes is susceptible to distortion by transformational interference voltages as well as line noise potentials.
In more recent times, magnetoinductive flowmeters have generally been designed to work with a switched continuous field. A switched continuous field of that type is produced by feeding the field coils of the magnet a current essentially with a time-based square-wave pattern whereby its polarity is periodically alternated. But equally possible is the use of a pulsating continuous field that is maintained by periodically feeding to the field coils of the magnet a time-based square-wave current of always the same polarity. However, in a preferred method the field current is periodically polarity-reversed, thus producing a periodically alternating magnetic field, because changing the polarity of the magnetic field suppresses interference signals such as electrochemical noise. The voltage between the measuring electrodes, being proportional to the flow rate, is usually quite low, typically in the microvolt range. Measuring that voltage requires high resolution (approx. 100 nV); in the traditional magnetoinductive flowmeters that employ the switched constant-field principle, the measuring frequency is in the 1 to 100 Hz range.
In these earlier magnetoinductive flowmeter designs, the voltage collected at the measuring electrodes is usually fed to a preamplifier before the preamplified voltage signal, being proportional to the flow rate, can be processed further. Widely used preamplifiers are of the differential-amplifier variety which are typically operated with a supply voltage of ±15 V. The dynamic range, meaning the highest possible output voltage of the preamplifier, is thus 15 V, i.e. +15 V for positive signals and −15 V for negative signals. When a preamplifier is operated with a supply voltage of ±15 V, the reference electrode of the magnetoinductive flowmeter is generally held at a potential of 0 volts, i.e. earth potential.
Desirably, however, it should also be possible to operate preamplifiers for magnetoinductive flowmeters at a lower supply voltage, for instance 5 V (0 V, +5 V). Such an amplifier would, in essence, provide a dynamic range of 0 V to 5 V. Appropriate analog components permitting a supply voltage-range from 0 V to 5 V have by now become available, highly precise sigma/delta converters are capable of working with these low input voltages and, most of all, power dissipation is minimized.